With latest iOS 10,Apple add new feature for third party developer called iMessage apps.Using that developer can make own apps. Apple has added new section [iMessage apps] at iTunes.This is only related iMessage.iMessage apps will only be available on iOS, their content will still be viewable on macOS and watchOS devices. iMessage extension is similar things like photos ,keyboard etc.. extension.Only different is that iMessage App Store exists on its own inside the Messages app, you can create an iMessage app without having to create an iOS app that goes on the user's home screen. - View it on GitHub